The present invention relates to an A.C. switching circuit which is inserted between an A.C. source and a load circuit and is capable of preventing an arc from being generated between contacts upon their opening or closing operation.
There has been suggested one of the A.C. switching circuits of the kind referred to in, for example, German Pat. No. 1,161,618, but the circuit of this patent still has been defective in the following respects. According to the patent, a first relay switch is connected in series with an A.C. source and a load, a series circuit of a diode and second relay switch is inserted in parallel to the first relay switch, and the two relay switches are opened or closed respectively by a further relay which is driven by a flip-flop. However, it is difficult to control the opening and closing operations of the first and second relay switches at a proper timing. More specifically, the second realy switch is closed during each negative half cycle of the A.C. source current to apply a positive voltage to the diode so as to prevent the arc generation at the second relay switch, while the first relay switch is closed during each positive half cycle of the source current, upon which closing the arc generation is also prevented from occurring because of the same potential with the diode. Further, the first relay switch is opened during the positive half cycle of the source current and the second relay switch is opened during the negative half cycle to prevent the arc generation. However, this operation has the disadvantage of requiring the relay switches opened and closed in a very accurately timed relation. In addition, in the case where the relays are of latching type and D.C. source voltage restores from an interruption, it is necessary to initially reset the relays and to subsequently detect the state of the flip-flop, whereby the circuit arrangement has been made rather complicated.